1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to an in-plane switching mode LCD device.
2. Description of the Related Art
An LCD device of twisted nematic mode, which is mainly used in flat panel display devices having high image quality and low power consumption, has a narrow viewing angle. The refractive anisotropy of liquid crystal molecules together with the vertical orientation of the liquid crystal molecules with respect to the substrate when voltage is applied to a twisted nematic mode LCD device causes a narrow viewing angle. In contrast, an in-plane switching mode LCD has a wide viewing angle since the liquid crystal molecules are oriented in a direction parallel with the substrate when voltage is applied to an in-plane switching mode LCD device.
FIG. 1A is a plan view showing a unit pixel of a related art in-plane switching mode LCD. FIG. 1B is a cross-sectional view along line I-I′ in FIG. 1A. As shown in FIG. 1A, gate lines 1 and data lines 3 are respectively arranged in longitudinal and transverse directions on a transparent first substrate 10 to define a unit pixel area. In an LCD device with a panel of unit pixels, n gate lines 1 and m data lines 3 are crossed to make a panel having n×m unit pixels.
In the unit pixel area, a thin film transistor 9 is formed adjacent to where the gate line 1 and the date line 3 cross each other. The thin film transistor 9 includes a gate electrode 1a, a source electrode 2a and a drain electrode 2b that are respectively connected to the gate line 1, the data line 3 and the pixel electrode 7. As shown in FIG. 1B, a gate insulating layer 8 is formed above the gate electrode 1a. A semiconductor layer 5 is formed above the gate insulating layer 8. The source electrode 2a and the drain electrode 2b are formed in contact with respective sides of the semiconductor layer 5.
A common line 4 is arranged to be parallel with the gate line 1 and traverses through the unit pixel area. As shown in FIG. 1A, the common electrode 6 and the pixel electrode 7 are arranged to be parallel with each other such that the orientation of the liquid crystal molecules can be changed. The common electrode 6 is formed simultaneously with the gate electrode 1a and connected to the common line 4. The pixel electrode 7 is formed simultaneously with the both source electrode 2a and drain electrode 2b such that the pixel electrode 7 is connected to the drain electrode 2b of the thin film transistor 9. A passivation layer 11 and then a first alignment layer 12a are formed over the first substrate 10, including the source/drain electrodes 2a and 2b. 
As shown in FIG. 1B, a branch of the common electrode 6 is formed adjacent to the periphery of the unit pixel area to shield the pixel electrode 7 from the electric field generated between the pixel electrode 7 and the data line 3 on the periphery of the unit pixel area. Also, a pixel electrode line 14, which overlaps the common line 4, forms a storage capacitor using the gate insulating layer 8 between the pixel electrode line 14 and the common line 4 as the insulator of the storage capacitor. The width W of the common line 4 that traverses across the unit pixel area under the pixel electrode line 14 should be sufficiently large to ensure a sufficient storage capacitance for the time needs of the LCD device.
A second substrate 20 includes a black matrix 21, a color filter 23 and a second alignment layer 12b. In particular, light leakage from the unit pixel area is prevented by the black matrix 21 formed above the gate line 1, the data line 3 and the thin film transistor 9. A color filter 23 is formed adjacent to the black matrix 21 on the second substrate 20. A second alignment layer 12b is provided on the color filter 23. A liquid crystal layer 13 is provided between the first and second substrates 10 and 20.
When a voltage is not applied to the in-plane switching mode LCD device in FIGS. 1A and 1B, the liquid crystal molecules in the liquid crystal layer 13 are oriented in accordance with the alignment direction of the first and second alignment layers 12a and 12b. However, when a voltage is applied between the common electrode 6 and the pixel electrode 7, the liquid crystal molecules are reoriented to be parallel with the substrate and vertical to the extended direction of the common electrode 6 and the data line 3. Since the liquid crystal molecules in the liquid crystal layer 13 are always reoriented on the same plane, inversion of gray level is not generated in up-and-down and left-and-right viewing angle direction.
In the in-plane switching mode LCD device of FIGS. 1A and 1B, the common electrode 6 and the pixel electrode 7 may be formed of an opaque metal in the unit pixel area. The common electrode 6 should be formed adjacent to the periphery of the unit pixel area, and therefore, light-transmission regions of even numbers are created. For example, four light transmission areas are shown in FIG. 1. Accordingly, the aperture ratio is reduced. Further, even though the common electrode is adjacent to the periphery of the unit pixel area, there is a limit as to how much data voltage can be shield from the pixel electrode during driving.